PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Late Southern Fall (Ls = 86˚, 2 PM)Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29PTYS 554Evolution of Planetary SurfacesAeolian Processes IIAeolian Processes IIPYTS 554 – Aeolian Processes II2Aeolian Processes IEntrainment of particles – settling timescalesThreshold friction speedsSuspension vs. saltation vs. reptation vs. creepDependences on gravity, densities of particle/airAeolian Processes IIMigration ratesDune typesDunefield pattern formationRipples vs. dunesVentifact, yardang erosionDust-devils and wind streaksPYTS 554 – Aeolian Processes II3Reminders from last lecture…Surface shear stress from the wind is often expressed as a shear velocity:Law of the wall predicts logarithmic wind velocity (u) changes with height (z):Where z0 is a surface roughness length scaleBalancing particle weight and wind shear stress we can calculate (with an empirical constant, A) the threshold shear stress or velocity:A2 more commonly called θ in fluvial studiesA also a function of fluid:particle density ratio ‘A’ is a function of Reynolds number so that very small particles (hiding in the laminar sub-layer) are harder to mobilize than larger particles.A preferred particle size exists that is sand-sized u*=tra uu*=1klnzzoæ è ç ö ø ÷ tT=A2rs- ra( )gdéëùû u*T=Ars- raraæ è ç ö ø ÷ gdPYTS 554 – Aeolian Processes II4One core expression for mass flux (q)There are many variations fit to empirical dataGreeley, 1985q»ragu*3PYTS 554 – Aeolian Processes II5Sand tends to stick in sandy areas and bounce quickly across rocky surfacesLow hills of sand feel asymmetric shear stressTurbulent shear stress at many wavelengths combined linearlyJackson and Hunt 1975Doesn’t work so well when slopes > 10-15°Shear stress drops to zero when slopes exceed ~ 25° (back-pressure effects)Pelletier 2009PYTS 554 – Aeolian Processes II6Formation of a dune…Asymmetric shear stress causes lee-slope of all low hills to steepen Lee slopes steepen past 25°Air flow separates from lee-slope surfaceSand falls onto the upper lee-slope and is not removed causing steepeningLee slope approaches angle of reposeSlope controlled by gain avalanchesDune advancesStossSlopeNegative mass balanceLeeSlopePositive mass balanceFlatNeutral mass balance qµ u*3µ t32PYTS 554 – Aeolian Processes II7Dune velocityMass sand flux: qSome fraction (f) of this comes from the dune itselfDune Velocity:Bigger dunes move slower for a given sand fluxDunefields with similar sized dunes are stableDune height is self-limiting due to streamline compressionhDxDt=2 f qrduneæèçöø÷h- 1PYTS 554 – Aeolian Processes II8Upwind: Stoss sideDownwind: Lee sideHighest point: crestSlipface start: BrinkDunesStossLeeHornHornPYTS 554 – Aeolian Processes II9Unidirectional windsDune shape depends on sand supplyBarchan Barchanoid ridges TransverseIncreasing sand supplyPYTS 554 – Aeolian Processes II10Barchan dunes provide valuable wind direction measurementsPYTS 554 – Aeolian Processes II11Late Southern Fall (Ls = 86˚, 2 PM)Late Southern spring (Ls = 286˚, 4 PM)Surface geology can be tied to atmospheric modelingIndicates the dunes formed under current wind directionsLate Southern spring (Ls = 286˚, 4 PM)Fenton et al.PYTS 554 – Aeolian Processes II12When wind directions varyLinear dunes – a little variationStar dunes – a lot of variationStar Linear/LongitudinalPYTS 554 – Aeolian Processes II13Longitudinal dunes on TitanLorenz et al., 2006PYTS 554 – Aeolian Processes II14Titan’s dunes cover a significant portion of the bodyWinds blow from west to east… opposite to what models predictRadibaugh & LorenzPYTS 554 – Aeolian Processes II15For fast dune (large disparity in dune size) collisions…A perturbation to a dune size leads to runaway growthDune interactions and dunefieldsPYTS 554 – Aeolian Processes II16CollisionsDunes have a separation bubble(zone of zero shear stress)Allows upwind dune to take sand from the downwind duneDune-size ratio close to 1Interaction is slowUpwind dune steals sand until size ratio is reversedDownwind dune is now faster and escapesDune-size ratio far from 1Interaction is fastDunes mergeDiniega et al. 2010PYTS 554 – Aeolian Processes II17Dune field pattern formationA stable dune field has dunes all roughly the same sizeConsider Incoming size ratio vs. Outgoing size ratio in collisionsOutgoing size ratio > IncomingDunes are closer to equal sizedNext collision will make the size ratios even closer to oneEquilibrium reached when all the dunes are the same size (also means no more collisions)Outgoing size ratio < IncomingDunes are more different in size after a collisionIf size ratio is too small then collisions get too fast and mergers start happeningOutgoing size ratio = IncomingSoliton-like behavior (misleading analogy) where dunes appear to pass through each otherDuran et al. 2005Herrmann et al. 2005PYTS 554 – Aeolian Processes II18Form perpendicular to wind directionMuch more dynamic than dunesAsymmetric, 8-10 vs 20-30 degree slopesSeparation bubbles, but not slip facesWavelengths 0.5cm to 25m (mega-ripples)Typical wavelengths 7-14cm, heights 0.5-1cmRipple index: L / Height, typically ~18Wavelength related to grain-size(but also wind speed…)RipplescmDL75.08.63Pelletier 2009PYTS 554 – Aeolian Processes II19Bagnold again…Saltation path length ~ wavelengthHigh angle impacts erode material from Stoss sideLow angle impacts deposit material in shadow zoneCoarse-grained material tends to get concentrated on the crestsPYTS 554 – Aeolian Processes II20YardangsWind blown particles abrades surfaceErosion leaves elongated mounds as remnantsRequires strong, virtually unidirectional, wind.Eroded material must be consolidatedVentifactsElongated erosional marks on rocksUsually works on originally circular vesiclesUsed as paleo-wind direction indicators e.g. pathfinder landing siteAeolian ErosionOlder wind actionRecentwind actionPYTS 554 – Aeolian Processes